This invention is directed to a composition useful for making circuit board substrates and electrical connectors comprising a blend of a poly(ether sulfone) and polysulfone. Additionally this invention relates to a circuit board substrate and electrical connector made from such a blend.
U.S. Pat. No. 4,281,038 describes a non-conductive substrate (board) for a printed circuit made from a thermoplastic selected from polyphenyloxide, polyphenylsulfide, polyimide and polyethersulfone. The thermoplastic has dispersed therein discrete particles of a material such as iron, copper, compounds or iron and compounds of copper which have a coating of a bonding assistant such as a silicone resin, an organic silane and a silane ester.
Circuit boards are widely used in the electrical industry for radio, television, computers, appliances, industrial and electronic equipment. Printed circuit boards have been traditionally manufactured from a copper clad epoxy-glass laminate. When starting with this material the shape of the printed circuit board must first be routed out and the holes for mounting the components (e.g., transistors, resistors, integrated circuits, etc.) individually drilled. The board is then masked with photoresist, the circuitry imaged and the copper etched away from areas where it is not wanted. An alternative to this procedure is to injection mold the circuit board substrate with the holes in place. The molded substrate is then put through several adhesion promotion steps and plated with electroless copper according to standard technology, to produce the printed circuit board. In this case the substrate material is limited to thermoplastic resins with sufficient thermal stability and chemical properties to survive wave soldering. Also, savings may result with these injection molded circuit board substrates due to the elimination of considerable mechanical processing such as routing and drilling.
The critical parameters of a printed circuit board, from a soldering standpoint, are its heat distortion temperature, environmental stress crack resistance and thermal expansion coefficient. The higher a substrate's heat distortion temperature and environmental stress crack resistance to solder fluxes, the less likely it will blister or delaminate during soldering.
When a polysulfone based on bisphenol A is molded into a circuit board substrate it has a heat distortion temperature which is generally too low for soldering temperatures such as those encountered in wave soldering. A circuit board substrate molded from poly(ether sulfone) is resistant to specific etching solutions (e.g., chromic acid) and is thus harder to prepare suitable surfaces for plating. Additionally, circuit boards molded from poly(ether sulfone) do not have, in many instances, acceptable plateability. Acceptable plateability requires good adhesion of electrolessly plated copper to the substrate.